Figure 2.

XPS of the samples as-formed by PLA and further dwelled in water for days. XPS of the as-formed PLA sample (upper black trace) and the samples with further
dwelling in water for 1 (middle red trace) and 20 days (lower blue trace) showing
the binding energy change of (a) Cu 2p from 932.8 eV for Cu+ to 933.2 eV for Cu2+, (b) corresponding shift of O 1s from 530.4 to 529.0 eV, and (c) relatively inert Zn 2p of Zn2+ at 1,020.8 eV (cf. text).